Hot water in a cup. Put bladder nipple in cup for a few seconds, then carefully peel off (try not to stretch bladder too much). Clean bladder and rough a little bit with some fine sandpaper. You can then stick the same valve back on with "Sweeda" glue, or any good clear vinyl glue.
I've done a ton of old Fuels in the past like this, they never leaked again after.
p.s. This is a Best kite isn't it?...
p.p.s. If they were properly welded on a hot car wouldn't have this effect.
